The present invention relates generally to a process of heat pressing of dental restorations and more specifically to a plunger for use in a pressing furnace for transferring pressure from a furnace piston into an investment ring.
Dental materials include porcelain facings, veneers, bridges, inlays, crowns, and a multitude of other products. The first step of the casting of, for example, an inlay or a crown, is the preparation of a wax pattern. The cavity is prepared in the tooth and the pattern is carved, either directly in the tooth or on a die that is a reproduction of the tooth and the prepared cavity. If the pattern is made in the tooth itself, it is said to be prepared by the direct technique. If it is prepared on a die, the procedure is called the indirect technique. However the pattern is prepared, it should be an accurate reproduction of the missing tooth structure. The wax pattern forms the outline of the mold into which the alloy or ceramic is cast. Consequently, the casting can be no more accurate than the wax pattern, regardless of the care observed in subsequent procedures. Therefore, the pattern should be well adapted to the prepared cavity, properly carved, and the distortion minimized. After the pattern is removed from the cavity, it is surrounded by a material which forms an investment. This process is called investing the pattern.
Commonly used investment materials include gypsum, phosphate and silica-based materials. Preferably, silica-based investments fabricated from all or a high percentage of quartz or cristobalite are used as dental investment materials. After the investment material has hardened, the wax is eliminated, typically by heat to provide a mold cavity for forming the dental restoration. The investment includes a pouring channel which is formed by a sprue on the wax model. This provides a channel through which the dental materials are supplied to the mold cavity. Dental materials, such as dental ceramics, may be inserted into a premolding space in the form of an unfinished piece or blank. The blank is softened by heat so that it can be introduced into the molding cavity in a viscous state using fairly low pressure to assume the shape of the mold cavity to form the desired dental prosthesis. This process is called heat pressing and is described in xe2x80x9cHot-Compressed Porcelain Process For Ceramo-Metal Restorationsxe2x80x9d by E. R. McPhee in Dental Porcelain: The State of the Art-1977, edited by Henry Yamada, USC School of Dentistry, Los Angeles, Calif. More recently, the process was described in an article by M. J. Cattel et al., entitled xe2x80x9cThe Biaxial Flexural Strength of Two Pressable Ceramic Systemsxe2x80x9d in Journal of Dentistry 27 (1999) 183-196.
A pressing furnace is used to press the ceramic material into the mold cavity and conform the material to the shape of the cavity. The pressing furnace includes a driving plunger (herein referred to as the xe2x80x9cinternal plungerxe2x80x9d) that contacts a second or external plunger inserted into a cylindrical mold which is made from refractory investment. This cylindrical mold is known as an investment ring in the dental field. The external plunger is in contact with the ceramic material. The external plunger transmits the pressing force from the driving plunger to the ceramic material and forces the material through the channel to the cavity. After the pressing operation, the internal plunger is raised, the investment ring and the external plunger are removed from the pressing furnace, and the resultant dental material is removed from the mold. The external plunger must be cleaned after the pressing operation.
In current practice, external plungers are fabricated of a high density alumina. The alumina plungers are intended to be reused for numerous pressings and are relatively expensive. One example of alumina used to manufacture plungers is AD-998 available from Coors Ceramics Company, Golden, Colo. AD-998 comprises 99.8% alumina and has a density of 3.92% gms/cc. It exhibits a flexural strength (MOR) of about 375 MPa (54,000 psi) and a Knoop hardness of 14.1 and a Rockwell hardness of 83 GPa. The coefficient of thermal expansion is 8.2xc3x9710xe2x88x926/xc2x0C. which is generally lower than the coefficient of thermal expansion of the materials to be pressed. Current pressable dental materials have coefficients of thermal expansion in the range of about 17 to about 18xc3x9710xe2x88x926/xc2x0C. for leucite-reinforced materials such as Empress(trademark) material available from Ivoclar, Amherst, N.Y. and OPC(copyright) material available from Jeneric/Pentron Inc., Wallingford, Conn. and in the range of about 10 to about 11 for lithium disilicate materials. The plunger must be sufficiently strong and durable to withstand repeated pressings.
Since the external plunger is in direct contact with the ceramic material, it may react or adhere to the ceramic material. Consequently, cracks may form, originating at the interface between the plunger and the ceramic or porcelain button, and propagating through the pressed shape. One reason for this is the difference in shrinkage and cooling rates between the plunger and the ceramic materials used to form the dental restoration. Moreover, if the ceramic materials stick to the plunger, cleaning becomes difficult, rendering reuse inconvenient and problematic. Removal of the glass-ceramic from the plunger requires grinding, sandblasting, and/or soaking the plunger in acid resulting in usage of time and labor which could otherwise be spent on other more constructive tasks. Furthermore, this cleaning process can weaken and distort the plunger reducing the useful life of the plunger. Other techniques have been attempted to solve plunger-related problems, such as modification of the pressing cycle or usage of a massive metal block as a heat sink to promote fast cooling of the ring in an effort to avoid cracking. Nevertheless, these solutions have not been proven to be fully effective and cracking may still occur from time to time.
There is a need to reduce or eliminate time involved in cleaning the external plunger after completion of the pressing operation. It is desirable that cracking problems occurring during the pressing operation be reduced or eliminated.
These and other objects and advantages are accomplished by the invention comprising a disposable external plunger for a pressing furnace. The plunger is fabricated of a partially sintered ceramic material. The partially sintered material is not sintered to full density. The strength of the partially sintered material may be at least about 3 MPa and not exceed 20 MPa.
Examples of partially sintered refractory materials include refractory materials which can be further delineated to castable refractory materials. Refractory investment materials is a further subclass which can be used in the manufacture of the plungers herein described.
Refractory investment materials useful herein include gypsum and phosphate-bonded investment materials which comprise a filler material such as silica materials, for example, quartz or cristobalite or a combination thereof. The plunger is fabricated by known casting methods. The mold used for making the plunger can be supplied by a manufacturer of molds or can be easily made by using an alumina or similar plunger as a model.
As an alternate embodiment herein, the plunger may be fabricated of an unfired ceramic material in the form of a green body. The green body has a sufficient green strength to withstand handling. The xe2x80x9cgreenxe2x80x9d plunger is partially sintered in the premolding phase of the pressing operation providing strength sufficient for pressing the pellet material in the fabrication of dental restorations.
The plunger herein provides an efficient and effective way to alleviate problems associated with prior art plungers. It is inexpensive and simple to manufacture and therefore can be disposed of after use. Laborious cleaning and grinding are not required with plungers herein described. Cracking problems that occur with prior art plungers are decreased, if not completely eliminated.